Method for pyrolytic production of semiconductor material



M. HEIM March 15, 1966 METHOD FOR PYROLYTIC PRODUCTION OF SEMICONDUCTORMATERIAL Filed NOV. 27. 1951 2 Sheets-Sheet 1 March 15, 1966 HEIM3,240,623

METHOD FOR PYROLYTIC PRODUCTION OF SEMICONDUCTOR MATERIAL Filed Now).27. 1961 2 Sheets-Sheet 2 Fig. 9

United States Patent Ofiice 3,249,623 Patented Mar. 15, 1966 3,240,623METHOD FGR PYRULYTEC PRODUCTION OF SEMICONDUCTOR MATERIAL Max lilleim,Munich, Germany, assignor to Siemens &

Halslre Alktiengesellschatt, Berlin, Germany, a corporation of GermanyFiled Nov. 27, 1961, Ser. No. 155,030 Claims priority, applicationGermany, Nov. 30, 1960, 8 71,471 4 Claims. (Cl. 117106) My inventionrelates to the pyrolytic production of semiconductor material, and hasas its general object the improvement of the etficiency and economy ofsuch production by reducing the processing time required and affording,within the same equipment, the simultaneous production of a much largernumber of products than heretofore producible in a single operation.

More particularly, my invention relates to a method and apparatus forthe production of semiconductor material, particularly silicon, bythermal dissociation of a gaseous compound of the semiconductor materialand precipitation of the semiconductor material onto a multiplicity ofcarrier or core rods consisting of the same material directly heated bypassing electric current through the rods.

The semiconductor rods employed as carriers or cores in pyrolyticproduction methods, being of extreme purity, have an extremely highspecific electric resistance when in cold condition and becomeappreciably conductive to electric current only at or near incandescenttemperatures. Consequently, for maintaining the core rods by directpassage of electric current at the high temperature required forpyrolytic decomposition and precipitation of the semiconductorsubstance, particularly silicon, it is necessary to first preheat thecore rods until they become capable of carrying the electric currentrequired for further maintenance and further increase of theirtemperature.

According to my invention, the heating, of the cores or carriers,required for reducing the specific electric resistance, prior tocommencing the pyrolytic production process proper, is obtained by firstfilling the reaction vessel, in which the core rods are mounted, with aprotective gas of relatively good heat conducting properties,particularly hydrogen, and by placing heater elements as close asfeasible to the carrier rods, or at least to a number of those rodsaccommodated in the processing vessel. By means of these heaters, thecore rods, or at least part of them are heated to a temperaturesufficient for these rods to assume an electric conductance adequate formaintaining a flow of electric heating current through the rods.Thereafter the heater elements are inactivated or switched-oft and aremoved out of the reaction chamber into an adjacent space, such as into acontainer filled with air or protective gas, whereafter the reactiongas, containing the semiconductor compound to be pyrolytically reducedand precipitated, is supplied to the reaction chamber while passingelectric current through the rods.

In the production of hyperpure semiconductor material from the gaseousphase, the preheating necessary for reducing the initially high specificresistance of the carrier rods heretofore has been effected with the aidof infrared radiators located outside of the reaction vessel consistingof quartz.

By comparison my invention achieves a number of advantages. In the firstplace, the transfer of heat from the source to the core or carrier rodsis not exclusively due to radiation but is also effected by heatconductance so that the carrier rods can be heated up to the temperaturerequired for good current conductance within a much shorter period oftime than heretofore required. Furthermore, the reaction vessel can begiven any desired large size because it is no longer necessary to use aquartz vessel which in practice can be made to a maximum diameter ofabout 180 mm. Thus, the invention also affords the use of processingvessels consisting of metal and provided with cooling means. Inconsequence, the invention permits simultaneous precipitation ofsemiconductor material upon a very large number of carrier rods, forexample about rods. In this manner, very large quantities of hyperpuresemiconductor material can be produced in a single operation. It hasalso been found that the economy and etficiency of the pyrolyticproduction method increases with the number of carrier rodssimultaneously employed because the consumption of electric current percarrier is considerably reduced and a better efiiciency or yield of thereaction gas being supplied is obtained.

In the method according to my invention, it is not necessary to preheatall of the carrier rods by respective heater elements, but it issufficient to preheat only part of the carrier rods. These heated rodsthen transfer the heat by radiation or conductance or both to theremaining number of carrier rods, said remaining rods being mounted at asufficiently small spacing from the initially heated carriers, so thatsaid remaining carrier rods are brought to the temperature required forsufiiciently good electric conductance.

For further description, reference will be made in the the following topreferred embodiments of apparatus according to the inventionillustrated by way of example on the accompanying drawings, in which:

FIGS. 1, 2 and 3 illustrate a first embodiment of a pyrolytic processingapparatus in three different stages of operation respectively.

FIG. 4 illustrates in vertical section another modification of suchapparatus.

FIG. 5 shows partly in section a portion of a further embodiment of suchapparatus;

FIGS. 6 and 7 are schematic sectional views of an embodiment ofpyrolytic production apparatus in two different stages of operationrespectively and utilizing the portion shown in FIG. 5

FIG. 8 illustrates two pairs of rods together with their electricalconnections; and

FIG. 9 schematically shows some of the heater elements with theirelectrical connections.

The same reference numerals used in the different figures are used todenote the same feature. The apparatus shown in FIGS. 1 to 3 comprises atable structure With a horizontal supporting ring 12. When preparing theapparatus for pyrolytic production of silicon, a bell-shaped hoodportion 21 is lifted off the table structure to make the top surface ofthe supporting ring 12 accessible. Then the rod-shaped carriers ofsemiconductor material such as hyperpure silicon are mounted on the ringstructure so as to form a peripheral row. Two of these carrier rods aredenoted by 11 and 11' in FIG. 1. The rods are secured at their lower rodend only to respective holders on the ring structure 12 and extend invertical upright positions. The holders, into which the lower ends ofthe rods are inserted, insulate these rods from the carrier structure12. The rods are spaced from each other a small distance only in theperipheral direction and each two or more of them are connected at thetop by an electrically conducting bridge piece of the same hyperpuresemiconductor material. This is shown in greater detail in FIG. 8 fortwo pairs of mutually adjacent carrier rods 11 and 11' mounted inrespective insulated holders 81 and 81 on the supporting ring 12 andconnected at their top ends by bridge pieces 23. During pyrolyticoperation of the apparatus the reaction chamber, located within the hood21 is tightly closed by a cover 13 which is mounted on a ported guiderod 14 and thereby displaceable in the vertical direction. Mounted onthe bottom side of the cover 13 and downwardly suspended therefrom areelectric heating elements in a concentric annular arrangement. Two ofthese heating elements are shown in FIG. 1 and denoted by 16 and 16.Each heating element may consist of a rod-shaped heating resistor asused in electric furnaces. That is, each of the resistance elements 16,16' is insulated from the cover plate 13 and connected in an electricheating circuit (FIG. 9) which, when energized by closing switch S16,causes the heating elements to be heated to incandescent temperature.Each heating element may also be formed by a heater winding embedded ina metal pipe, such tubular electric heaters being of the kind used forexample in conventional hot plates, stoves and baking ovens. If desired,the upper side of the cover 13 may be equipped with another concentricring arrangement of semiconductor carrier rods of which two are shown inFIG. 1 and denoted by 15 and 15', these rods consisting of the samehyperpure material, such as silicon as in the rods 11, 11, to constituterespective cores for the pyrolytic precipitation process.

During pyrolytic precipitation, the heater elements 16, 16' are loweredinto a cupshaped tank 17 which is gastightly joined at its rim with thecarrier ring 12. The ring 12 and the cover 13 are preferably providedwith respective cooling devices such as tubular coils traversed bycooling water (not shown). A gasket or sealing ring is located at 18between the supporting ring 12 and the cover 13 so that during pyrolyticoperation, with the cover 13 in the position shown in FIG. 1, a hermeticclosure of the reaction space within the hood 21 relative to theinterior of the tank 17 is secured.

After inserting the core rods and the heating elements into the ring 12and the cover 13, the hood 21 is lowered onto the carrier ring 12, andthe reaction chamber now enclosed within the hood 21 is filled with aprotective gas, for example hydrogen. The protective gas can beintroduced through fitting 14a to the hollow guide rod 14 and thenceagainst baffle plate 13' which distributes the gas over the rods in thereaction space. The reaction gas can also be introduced in the samemanner. The hood 21 is provided with a cooling coil 22 for maintainingit at relatively low temperature during pyrolytic precipitation. Theresidual waste gases are withdrawn through connection 21a.

At the commencement of the manufacturing process, the guide 14 isshifted to the position shown in FIG. 2. The tank 17, by means ofconnection 17a, is preferably filled with the same gas that is alsocontained in the reaction chamber proper. The peripheral row of heaterelements 16, 16' is now located close to the outer row of the siliconcore rods. Now, the heater elements which are provided with thenecessary current supply leads (shown in FIG. 9) are electricallyheated. The core rods are then heated substantially along their entirelength by heat radiation and also by heat conductance since thesurrounding gas is of good heat conductance and consists, for example,of hydrogen.

As mentioned, the core rods are likewise provided with current supplyleads which are connected for example to the rod holders 81 and 81' ofeach two adjacent rods 11, 11 as shown in FIG. 8, thus being connectedthrough resistors R11 with a voltage source when switch S11 is closed.When during the heating-up performance the core rods have assumed such ahigh temperature that the current passing through the core rod assumes avalue sufficient for heating the core rod up to a higher temperaturewithout additional heat from the heater elements, the heater elementsare switched-off and the cover 13 on guide rod 14 is lowered to theposition shown in FIG. 3 so that the reaction chamber 32 within hood 21is hermetically separated from the space in tank 17 as apparent fromFIG. 3. If the cover 13 is further equipped on its upper side withadditional core rods 15, 15 in an annular arrangement concentric to thatof the outer core rods 11, 11, then the inner row of core rods 15, 15'now also becomes electrically conductive due to heat transfer from theouter core rods 11, 11 and assumes the same incandescent temperature asthe rods in the outer row.

Thereafter, the gaseous semiconductor compound to be decomposed andprecipitated, for example silico-chloroform, is supplied to the reactionchamber 32, preferably together with hydrogen and under the influence ofthe high temperature of the silicon core rods, which are preferablybetween 1,000 and 1,200 C., and is dissociated with formation of siliconwhich precipitates onto the carrier rods, thus thickening theirdiameter. When using monocrystalline core rods, thick rod-shapedmonocrystals can be grown in this manner.

In the modified apparatus according to FIG. 4, the above-described tank17 is eliminated. The ring-shaped arrangement of heating elements issuspended from a plate 13. The hood 21 carries a massive partitioningwall 41 which, when the plate 13 is raised to the illustrated topposition, provides, with the aid of the peripheral gasket (not shown),for a hermetic seal between the reaction chamber 46 and the upper space47 in which then the heater elements 16, 16' are located. The core rods.11, 15 are mounted on a supporting plate 42 in two concentric ringarrangements of holders 43 and 44. For starting the pyrolytic operationby preheating the core rods, the cover plate 13 with the suspendedheater elements 16, 16' is lowered so that the rod-shaped heaterelements are located between the two concentric annular rows of corerods and both rows are simultaneously heated to the temperature requiredfor good conductance of the electric current. Thereafter, the plate '13with the heater elements is lifted to the position shown in FIG. 4 andthe pyrolytic operation proper is commenced.

FIG. 5 shows an enlarged portion of another apparatus for performing themethod. The complete apparatus embodying FIG. 5 is being describedhereirrbelow with reference to FIGS. 6 and 7. The enlarged portionaccording to FIG. 5 serves to provide for a hermetic seal of thereaction chamber by closure means connected with the heater elements.Several rod-shaped heater elements are grouped in parallel to a cluster51 and fastened in a holder ring 52. A valve cone 53 is mounted on a rod66 which coaxially traverses the cluster of heater elements and ispovided with spring means 67 tending to hold the disc 53 in closingposition. When the holder ring 52 is lowered to the position shown, thevalve cone 53 closes a conical bore in a carrier plate 54 on which theholders 55 (for the core rods are mounted, of which two are shown inFIG. 5 and denoted by 56 and '56.

FIG. 6 shows a processing apparatus provided with closure meansaccording to FIG. 5. The apparatus is shown to comprise five concentricannular rows of semiconductor 'core rods denoted by 61 to 65respectively. The holder ring 52 is shifted to raised position and theclusters of heater elements mounted thereon are located between theannular rows 63 and 64 to simultaneously heat both rows. After the corerods in these rows become conductive for sustaining and continuing theheating by means only of the electric current passing through theserods, the carrier ring 52 is lowered and the ceiling of the reactionchamber 71 then takes place by the means and in the manner describedabove with reference to FIG. 5. It is preferable to provide theapparatus with a tank 72 in order to provide a seal for the guide 73 ofthe carrier plate 52 and also for preventing the ingress of air or othergas during lowering of the heater elements and for preventing the influxof air in the event of leakage at the closure valves 53.

FIG. 7 shows the apparatus of FIG. 6 with the carrier ring 22 in loweredposition. Due to heat transfer from the incandescent rows 63 and 64 tothe inner rows 61, 62 and the outer row 65, all core rods become heatedto the temperature necessary for good current conductance. Thereafter,the reaction gas mixture is introduced into the reaction chamber 71 bysuitable means (not shown) similar to that of FIGS. 1 to 3. Formeasuring the corerod temperature during pyrolytic precipitation, thewatercooled bell '74 of the apparatus is provided with observationwindows 75.

While reference is made in the foregoing to the pyrolytic production ofsilicon, the method and apparatus according to the invention is alsoapplicable for the production of other semiconductor material, forexample, germanium. In this case, the carrier rods also consist ofgermanium and the gaseous mixture to be dissociated consists essentiallyof a gaseous germanium compound, particularly a germanium halogenidesuch as germanium chloroform.

Upon a study of this disclosure, it will be obvious to those skilled inthe art that with respect to various details in performance andequipment, my invention permits of various modifications and hence canbe given embodiments other than particularly illustrated and describedherein, without departing from the essential features of my inventionand within the scope of the claims annexed hereto.

I claim:

1. The method of producing semiconductor material by pyrolytic reductionand precipitation of the material from a gaseous compound thereof onto amultiplicity of core rods of the same material heated by electriccurrent passing through the rods, which comprises the steps of (a)filling the reaction space, prior to introduction of the gaseoussemiconductor compound, with a heatconducting non-oxidizing gas when therods are still in cold condition, and placing theater elements close toat least part of the rods;

(b) heating the rods by means of said heater elements to a temperatureat which the heated rods have sufficient electric conductance to bethereafter heated by passage of electric current through the rods;

(c) then discontinuing the heating by said heater elements and movingthe elements out of the reaction space;

(d) and thereafter admitting the gaseous compound of the semiconductormaterial to the reaction space and performing the pyrolytic production.

2. The method of producing silicon by pyrolytic p-recipitation from areaction gas, containing a silicon halogen compound and hydrogen, ontosilicon rods heated by electric current passing through the rods, whichcomprises the steps of (a) filling the reaction space, prior tointroduction of the gaseous semiconductor compound, with aheatconducting non-oxidizing gas when the rods are still in coldcondition, and placing separate electric heater elements in proximityand substantially along the entire length of at least part of the rods;

( b) heating the rods by means of said heater elements to a temperatureat which the heated rods have sufiicient electric conductance to bethereafter heated by passage of electric current through the rods;

(0) then discontinuing the heating by said heater elements and movingthe elements out of the reaction space into a gas-filled adjacent space;

(d) and thereafter passing the reaction gas into the reaction space andperforming the pyrolytic production.

3. The method of producing semiconductor material by pyrolytic reductionand precipitation of the mate-rial from a gaseous compound thereof ontoa multiplicity of core rods of the same material heated by electriccurrent passing through the rods, which comprises the steps of (a)filling the reaction space, prior to introduction of the gaseoussemiconductor compound, with a heat-conducting non-oxidizing gas whenthe rods are still in cold condition, and placing heater elements closeand along only some of the semiconductor rods;

(b) applying to all rods an electric voltage sufiicient to subsequentlymaintain the rods at pyrolytic temperature by electric current driven bysaid voltage through said rod once the rods are at an incandescenttemperature;

(c) electrically operating said heater elements to thereby first heatsome of said semiconductor rods to said incandescent temperature andthereafter permit said first heated rods to also heat all other rods tosaid temperature;

(d) then discontinuing the heating by said heater elements and movingthem out of the reaction space while continuing the heating of allrods-by said current;

(e) and thereafter admitting the gaseous compound of the semiconductormaterial to the reaction space and performing the pyrolytic production.

4. The method of producing semiconductor material by pyrolytic reductionand precipitation of the material from a gaseous compound thereof onto amultiplicity of core rods of the same material heated by electriccurrent passing through the rods, which comprises the steps of (a)filling the reaction space, prior to introduction of the gaseoussemiconductor compound, with a heatconducting non-oxidizing gas when therods are still in cold condition, and placing heater elements close toat least part of the rods;

(b) heating the rods by means of said heater elements to a temperatureat which the heated rods have sufficient electric conductance to bethereafter heated by passage of electric current through the rods;

(c) then discontinuing the heating by said heater elements and movingthe elements (from the reaction space (d) into an adjacent gas-filledspace, and continuing the heating of the rods :by only passing electriccurrent therethrough;

(e) sealing the reaction space from the adjacent space now containingthe heater elements;

(f) and thereafter admitting the gaseous compound of the semiconductormaterial to the reaction space and performing the pyrolytic production.

References Cited by the Examiner UNITED STATES PATENTS 2,196,767 4/ 1940Hasche 23277 2,986,451 5/1961 Wilson 117-406 3,010,797 11/1961 Aries23-223.5 3,011,877 12/ 1961 Schweikert et al 23- 2-84 3,023,087 2/1962Enk et al 23--223.5 3,063,871 11/1962 Barkemeyer 117107 X 3,128,1544/1964 Bean et al. 2'3-223.5 3,140,922 7/ 1964 Sterling 23-22353,147,141 9/1964 Ishizuka 23277 X JOSEPH B. SPENCER, Primary Examiner.

MAURICE A. BRINDISI, RICHARD D. NEVIUS,

Examiners.

1. THE METHOD OF PRODUCING SEMICONDUCTOR MATERIAL BY PYROLYTIC REDUCTIONAND PRECIPITATION OF THE MATERIAL FROM A GASEOUS COMPOUND THEREOF ONTO AMULTIPLICITY OF CORE RODS OF THE SAME MATERIAL HEATED BY ELECTRICCURRENT PASSING THROUGH THE RODS, WHICH COMPRISES THE STEPS OF (A)FILLING THE REACTION SPACE, PRIOR TO INTRODUCTION OF THE GASEOUSSEMICONDUCTOR COMPOUND, WITH A HEATCONDUCTING NON-OXIDIZING GAS WHEN THERODS ARE STILL IN COLD CONDITION, AND PLACING HEATER ELEMENTS CLOSE TOAT LEAST PART OF THE RODS; (B) HEATING THE RODS BY MEANS OF SAID HEATERELEMENTS TO A TEMPERATURE AT WHICH THE HEATED RODS HAVE SUFFICIENTELECTRIC CONDUCTANCE TO BE THEREAFTER HEATED BY PASSAGE OF ELECTRICCURRENT THROUGH THE RODS (C) THEN DISCONTINUING THE HEATING BY SAIDHEATER ELEMENTS AND MOVING THE ELEMENTS OUT OF THE REACTION SPACE; (D)AND THEREAFTER ADMITTING THE GASEOUS COMPOUND OF THE SEMICONDUCTORMATERIAL TO THE REACTION SPACE AND PERFORMING THE PYROLYTIC PRODUCTION.